Salicylaldehyde is an industrially important compound as an intermediate chemical for the preparation of perfumes, pesticides, chelating agents, etc.
The following methods are, for example, hitherto known as the preparation of hydroxybenzaldehyde:
(1) So-called Reimer-Tiemann Reaction: PA0 (2) Method proposed by Yoel Sasson et al. [Tetrahedron Lett., 3753 (1979) and 1875 (1980)]:
In accordance with this reaction, a phenol is reacted with chloroform and an aqueous alkali solution in a heterogeneous system to prepare a mixture of salicylaldehyde and p-hydroxybenzaldehyde. Generally, in this method, the yield of salicylaldehyde is low. Further, an excess amount of chloroform over the phenol is necessary, and recovery and recycling of unreacted chloroform and phenol are not necessarily easy (and expensive).
There is also known a Dow method as one of the typical examples of improved Reimer-Tiemann reaction method, wherein a phenol, chloroform and an alkali metal hydroxide are reacted in a solvent medium consisting of aqueous methanol containing 10 to 75% by weight of methanol (as disclosed in U.S. Pat. No. 3,365,500). Even in this method, the conversion of the phenol is, however, low, and the separation and recovery of unreacted phenol are difficult. Further, the production ratio of salicylaldehyde and p-hydroxybenzaldehyde is lower than that obtained under the ordinary Reimer-Tiemann condition. Therefore, this Dow method accompanies difficulties in the separation of the components from the reaction mixture.
In accordance with this method, when an aliphatic tertiary amine is added to the reaction mixture obtained under the ordinary Reimer-Tiemann condition (where excess amounts of chloroform and an aqueous alkali solution are used over the phenol), the yield of salicylaldehyde is improved without any undesirable influence over the yield of p-hydroxybenzladehyde. But this method accompanies not a few disadvantages. Namely, the tertiary amine which can be used is limited to a few species such as (n-C.sub.4 H.sub.9).sub.3 N, etc., and a reaction to form an alkyl ether of phenol also takes place competitively. Further, excess amounts of chloroform and an aqueous alkali solution over the phenol need be used, which leads to the foregoing disadvantages. Still further, it has been experimentally confirmed by the present inventors under the same conditions as reported that the effects brought by the addition of the aliphatic tertiary amine are not so drastic as reported.
(3) A method in which a salicylaldehyde is prepared in good yield by one step reaction using as starting materials a phenol and a formaldehyde and a specific catalyst (a base represented by a tertiary amine and/or an organometallic salt):
This method is described in, for example, Japanese Patent Application (OPI) Nos. 34737/78 and 163538/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and J.C.S., Perkin I, 1862 (1980) and the like. This method, however, accompanies not a few disadvantages. In other words, an excess amount of formaldehyde over the phenol, a poisonous catalyst (e.g., SnCl.sub.2, SnCl.sub.4, Cr(acac).sub.3, etc.) or a fairly large amount of an organic amine as an additive need be used, and therefore, a complicated post-treatment and waste water treatment and the like are inevitable.